The present invention relates generally to integrated circuit (IC) packaging and specifically to IC packaging which provides a hermetic seal and reduces dose enhancement from secondary electrons.
In the design of IC packaging that will be used in radiation environments, attention must be given to designing the package so as to protect the circuits from damage due to radiation. Spacecraft or satellites traveling in typical earth orbits are exposed to a natural space radiation environment including gamma rays, x-rays and other high energy particles. Electrons and x-rays are the principal contributors to total dose radiation problems within a spacecraft or satellite. The type of interactions that occur between photons and the atoms in a bombarded or exposed material depends on the atomic number of the target material. X-rays become absorbed by a targeted atom and the atom emits an electron. For atoms having a high atomic number, such as gold, it is easier to liberate an electron than it is for atoms having a low atomic number.
The lid or cover of an IC package has an underside which is typically located near and facing the active region of the IC chip. Package lids are typically made of nickel/iron or kovar that has been nickel and gold plated. During package assembly the lid is typically brazed to the package with a gold/tin eutectic solder. Unfortunately the gold plated surface of the underside of the lid is facing the integrated circuit which will be a generator for electrons when the package is exposed to x-rays and other radiation. The active region of the chip is located near the top surface of the chip, and it is the dose in the active region which affects IC performance. The gold surface of the underside of the lid when bombarded with high energy particles liberates electrons to the chip surface. This electron showering of the integrated circuit can affect performance.
Ceramic lids do not contribute to total dose and a ceramic lid is sometimes used in lieu of the gold plated kovar lid previously described. The ceramic package would typically have a ceramic lid secured by means of solder through a solder reflow process. It is known that in radiation environments severe enough to produce thermal mechanical shock (TMS) to components, metal lid solders can melt and spatter over the IC die causing failure.
Thus a need exists for a package that does not contribute to total dose and can withstand TMS.